Submicron Sized Silicon Powder with Low Oxygen Content

ABSTRACT

A submicron sized Si based powder having an average primary particle size between 20 nm and 200 nm, wherein the powder has a surface layer comprising SiO x , with 0&lt;x&lt;2, the surface layer having an average thickness between 0.5 nm and 10 nm, and wherein the powder has a total oxygen content equal or less than 3% by weight at room temperature. The method for making the powder comprises a step where a Si precursor is vaporized in a gas stream at high temperature, after which the gas stream is quenched to obtain Si particles, and the Si particles are quenched at low temperature in an oxygen containing gas.

TECHNICAL FIELD AND BACKGROUND

This invention relates to submicron sized silicon based powders havinglow oxygen content and the synthesis of this powder using a gas phasetechnology.

Silicon powders are currently developed and used in a wide variety ofapplications including lithium-ion batteries, printed electronics andsolar applications. These applications require ultrafine powders withlow oxygen content.

Lithium-ion batteries are the most widely used secondary systems forportable electronic devices. Compared to aqueous rechargeable cells,such as nickel-cadmium and nickel metal hydride, Li-ion cells havehigher energy density, higher operating voltages, lower self dischargeand low maintenance requirements. These properties have made Li-ioncells the highest performing available secondary battery.

The worldwide energy demand increase has driven the lithium-ion batterycommunity to search for new generation electrode materials with highenergy density. One of the approaches is to replace the conventionalcarbon graphite negative electrode material by another better performingactive material, being a metal, metalloid or metallic alloy based onsilicon (Si), tin (Sn) or aluminum (Al). These materials can providemuch higher specific and volumetric capacity compared to graphite. Ontop of the specific composition of the negative electrode material, thesurface properties of the particles are playing a key role in theelectrochemical behaviour of the resulting Li-ion battery. Therefore, itis of paramount importance to be able to optimize those parameters inorder to enhance the electrochemical performances of the negativeelectrode.

The composite electrode needs to posses mixed conductivity with bothionic lithium and electrons. Such a complex medium is generally obtainedby mixing together the active material particles with differentadditives such as a very fine powder of carbon black and a polymericbinder. The binder additive has a complex role since it not only givesmechanical strength to the composite electrode but also allows for agood adhesion between the electrode layer and the current collector, andit gives the composite electrode a sufficient liquid electrolyte uptaketo provide internal ionic percolation.

As mentioned Si-based negative electrode materials could significantlyenhance the energy density of the commercial lithium ion batteries.Silicon has the largest theoretical gravimetric capacity (3579 mAh/g)corresponding to the following reaction: 15Li+4Si→Li₁₅Si₄ and a largevolumetric capacity (2200 mAh/cm³). However, the microscopic structureof these materials and their huge volume expansion upon lithiumintercalation had never allowed reaching acceptable life characteristicsfor their use in rechargeable cells. The synthesis of materials at thesubmicron scale allows to overcome the main drawbacks of these materialsand makes them suitable candidates for the replacement of carbon. Aninteresting method to prepare submicron powders is plasma technology, asis disclosed in WO 2008/064741 A1.

Unfortunately, these submicron silicon powders rapidly oxidize whenexposed to air. This uncontrolled oxidation of submicron sized siliconpowder finally results in oxygen contents above 10 wt %. This highoxygen level will have a negative impact on the electrochemicalbehaviour of these Si based powders in Li-ion batteries, generating highcapacity losses during first cycling (the so called irreversiblecapacity) because of the reduction of this layer.

It is an aim of the present invention to improve or even overcome theseproblems, and to provide for better negative electrode materials thatcan be manufactured by a simple and economical process.

SUMMARY

Viewed from a first aspect, the invention can provide a submicron sizedSi based powder having an average primary particle size between 20 nmand 200 nm, wherein the powder has a surface layer comprising SiO_(x),with 0<x<2, the surface layer having an average thickness between 0.5 nmand 10 nm, and wherein the powder has a total oxygen content equal orless than 3% by weight at room temperature. The surface layer may alsoconsist only of SiO_(x), with 0<x<2. In one embodiment the surface layeris thinner than 5 nm, in order to avoid large irreversible capacitiesduring first cycling and, in another embodiment it is thicker than 0.5nm in order to have a stable passivated powder that will not furtheroxidize when exposed to air or oxidizing gases. The thickness of theoxidized layer is herein expressed as the average thickness of the Layermeasured on transmission electron microscopy (TEM) pictures. The Sibased powder may consist of pure Si. The Si based powder may also benanosized, i.e. with an average primary particle size between 20 nm and200 nm.

hi one embodiment the submicron sized Si based powder has an oxidizedsurface layer comprising SiO_(x), with 1≤x<2. In another embodiment thepowder has a purity of at least 98 at % Si. The Si based powder may alsoconsist of pure Si. In another embodiment the submicron sized Si basedpowder has a total oxygen content less than 4% by weight after beingaged for 1 hour at 500° C. under atmospheric conditions and in air. Inyet another embodiment the submicron sized Si based powder has a totaloxygen content less than 5% by weight after being aged for 1 hour at700° C. under atmospheric conditions and in air. These conditionsguarantee that the passivation layer of the Si based powder is stableand further oxidation will not take place.

The submicron sized Si based powder described above can further comprisean element M selected from the group consisting of transition metals,metalloids, Group IIIa elements and carbon. In one embodiment Mcomprises either one of more elements of the group consisting of nickel,copper, iron, tin, aluminium and cobalt.

Viewed from a second aspect, the invention can provide the use of thesubmicron sized Si based powder as a negative electrode material in aLi-ion secondary battery.

Viewed from a third aspect, the invention can provide a method formanufacturing the Si based powder described above, comprising the stepsof:

-   -   providing a Si based precursor,    -   providing a gas stream at a temperature of at least 1727° C.        (equivalent to 2000K),    -   injecting the Si based precursor in the gas stream, thereby        vaporizing the Si precursor,    -   quenching the gas stream carrying the vaporized Si precursor to        a temperature below 1327° C. (equivalent to 1600K), thereby        obtaining submicron sized Si particles,    -   passivating the submicron sized Si particles in an oxygen        containing gas at a temperature below 700° C., and preferably        below 450° C. and    -   separating the Si particles from the gas stream.

Such a process yields a submicron sized Si based powder with a surfacelayer comprising a mixture of Si sub-oxides (SiO_(x), with x<2) by thecontrolled passivating step, combined with the other process steps.

In one embodiment the passivation step is performed at a temperaturebetween room temperature and 100° C. In another embodiment, the gasstream is provided by means of either one of a gas burner, a hydrogenburner, an RF plasma or a DC arc plasma. In yet another embodiment, thepassivation step is performed in an oxygen containing gas furthercomprising a secondary gas consisting of either one or more of the groupconsisting of Ar, N₂, H₂, CO and CO₂. In yet another embodiment, theoxygen containing gas is a mixture of oxygen and nitrogen, with lessthan 1% oxygen by weight. In a further embodiment, the passivation stepcan be carried out for a period of less than 60 minutes, and preferablyless than 10 minutes. In another further embodiment, the gas stream isprovided in a radio frequency inductively coupled plasma, and the gasstream comprises argon gas.

DETAILED DESCRIPTION

Submicron sized silicon based powder with a controlled oxygen level atthe surface can, when used as a negative electrode material in a lithiumion secondary battery, limit the first irreversible capacity of thisnegative electrode whilst maintaining a high reversible capacity, due tothe powder's small particle size and its corresponding large surfacearea, combined with a low oxygen content. The powder can consist ofsilicon particles covered with a very thin homogeneous layer of oxidizedmaterial, the particles having a total oxygen content less than 3 wt %at room temperature.

In one embodiment the silicon submicron powder has an average primaryparticle size of between 20 nm and 200 nm, where the average primaryparticle size (d_(av)) is calculated from the specific surface area,assuming spherical particles of equal size, according to the followingformula:

${d_{av} = \frac{6}{\rho \times {BET}}},$

in which ρ refers to the theoretical density of the powder (2.33 g/cm³)and BET refers to the specific surface area (m²/g) as determined by theN₂ adsorption method of Brunauer-Emmett-Telfer (BET technique).

The present invention can also provide a method for producing this Sibased powder whereby a Si based precursor is provided, a gas stream at atemperature of at least 1727° C. is provided, the Si based precursor isinjected in the gas stream, whereby the Si precursor is vaporized, thegas stream is cooled at a temperature below 1327° C., whereby submicronsized Si particles are obtained that are finally passivated in anoxidizing gas at a temperature below 700° C. The gas stream can beprovided by a radio frequency inductively coupled plasma, and the gasstream can comprise argon gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: TEM images (low magnification (A) en high magnification (B))showing the presence of a thin amorphous SiO_(x) layer at the surface ofSi submicron particles.

FIG. 2: Delithiation curves for Si powders with oxygen level of 2.8 wt %(full line) and 25.0 wt % (dotted line), Voltage vs. Li (V) againstCapacity (mAh/g)

FIG. 3: Oxygen level (in wt %—left axis—full line) and BET value(m²/g—right axis—dotted line) of Si submicron powder as a function ofaging temperature (° C.).

FIG. 4: Oxygen level (in wt %) of Si submicron powder as a function ofstorage time (in number # of days) in air at room temperature.

The invention may be practiced, for example, by way of the differentexamples described below.

EXAMPLE 1

A micron-sized Si powder is provided as Si precursor. A 60 kW radiofrequency (RF) inductively coupled plasma (ICP) is applied, using anargon plasma with 2.5 Nm³/h argon gas. The solid silicon precursor isinjected in the plasma at a rate of 220 g/h, resulting in a prevalent(i.e. in the reaction zone) temperature above 2000 K. In this firstprocess step the Si precursor is totally vaporized followed by anucleation into submicron sized Si powder. An argon flow of 10 Nm³/h isused as quench gas immediately downstream of the reaction zone in orderto lower the temperature of the gas below 1600 K. In this way the metalnuclei will be formed. Finally, a passivation step is performed at atemperature of 100° C. during 5 minutes by adding 100 L/h of a N₂/O₂mixture containing 0.15 mole % oxygen.

The submicron sized Si powder has a cubic crystalline phase and aspecific surface area of 40±2 m²/g (as measured by the BET technique),which corresponds to a mean primary particle size of about 60 nm.Chemical analysis shows that the oxygen content is 2.8 wt %, whilst TEMcharacterization shows the presence of a thin amorphous SiO_(x) surfacelayer with a thickness of 1-2 nm, as is shown in FIG. 1.

A paste is prepared by adding the obtained silicon powder to a 2% Na-CMCwater-based solution. Subsequently acetylene black is added. The finalpaste, having a silicon/CMC/acetylene black ratio of 50/25/25, isfinally ball milled for 30 minutes. Coatings with a thickness between 20and 30 μm are deposited on a copper foil by doctor blade coating. Thefirst drying of the paste was done using a conventional hot-air furnacebut can also be done at room temperature or using a vacuum oven, aconveyer furnace, drying on a heated surface, drying with infra-redirradiation, drying with far infrared irradiation, drying with inductionsystem, coating on a heated electrode, drying in a inert atmosphere. Thedrying method, temperature and sequence influence the stability of thepaste, the internal stress and possible cracking in the dried electrode.Finally coin cell type batteries are prepared in a glove box usingLi-foil as counter electrode. Battery tests are performed on theelectrodes with the following conditions: cycling between 0.01 and 1.0 Vat a rate of C/20, where C is defined as charging/discharging, at a rateof 3572 mAh/g per hour.

Table 1 gives an overview of the capacity of the 1^(st) delithiationstep. The value in the Table is an average for 3 coin cells. A capacityof 3700 mAh/g silicon is measured, and a very low irreversible capacityof less than 8% is obtained after the first cycle (Table 1 & FIG. 2).

COUNTER EXAMPLE CE 2

A silicon powder is produced in the 60 kW radio frequency (RF)inductively coupled plasma (ICP) as described in Example 1. Afterquenching however a modified passivation step is applied at atemperature of 500° C. during 5 minutes, by adding 150 L/h of a N₂/O₂mixture containing 0.15 mole % oxygen.

The powder has a cubic crystalline phase and a specific surface area of40±2 m²/g (as measured by the BET technique), which corresponds to amean primary particle size of about 60 nm. Chemical analysis shows thatthe oxygen content is 6.8 wt %, whilst TEM characterization shows thepresence of a thin amorphous SiO_(x) surface layer with a thickness of2-5 nm.

A paste is prepared and coin cells are made and tested as described inExample 1. A delithiation capacity of 3500 mAh/g silicon is measured,and a irreversible capacity of 573 mAh/g (14%) is obtained after thefirst cycle (see Table 1), which is considered too high.

COUNTER EXAMPLES CE 3-4

Two commercially available silicon samples were purchased, and oxygencontents of respectively 19.3 wt % (Counter example 3 obtained fromKaier, CN, with a BET value of 20 m²/g and an estimated average primaryparticle size of 130 nm) and 25 wt % (Counter Example 4 obtained fromAldrich, US, with a BET value of 34 m²/g and an estimated averageprimary particle size of 75 nm). The average thickness of the surfacelayer of Counter Example 3 is 15 nm (surface layer thickness and oxygencontent are related to each other). A paste is prepared and coin cellsare made and tested as described in Example 1. This results in lowdelithiation capacities of respectively 2800 and 1500 mAh/g silicon (seeTable 1). Furthermore, high irreversible capacity values of 600 mAh/g(17%)(CExample 3) and 644 mAh/g (30%)(CExample 4) are obtained after thefirst cycle, which is higher than for Example 1.

TABLE 1 Overview of coin cell testing results Delithiation First FirstOxygen capacity first irreversible irreversible Example content cyclecapacity capacity number (wt %) (mAh/g) (mAh/g) (%) 1 2.8 3700 305 7.6CE 2 6.8 3500 573 14.1 CE 3 19.3 2800 600 17.6 CE 4 25.0 1500 644 30.0

FIG. 2 shows the capacity (mAh/g) of the silicon in the electrodes ofthe coin cells of Example 1 and Counter example 4 for the first cycle.

EXAMPLE 5

The stability of the powder as function of time and temperature ischecked in stability experiments. The powder obtained in Example 1 isannealed in air at different temperatures for 1 hour and the oxygencontent of the resulting powders is measured by chemical analysis. It isillustrated in FIG. 3 that the oxygen level remains stable in air up to700° C., after which a drastic increase up to 50 wt % oxygen takesplace. In FIG. 3 the oxygen level (full line) is to the left in wt %,whilst the corresponding BET value (in m²/g—dotted line) is shown to theright, both as a function of the temperature in C.

At room temperature, no significant increase of the oxygen level as afunction of time is observed, as is illustrated in FIG. 4, where theoxygen level (in wt %) is shown against the time in number of days.

While specific embodiments and/or details of the invention have beenshown and described above to illustrate the application of theprinciples of the invention, it is understood that this invention may beembodied as more fully described in the claims, or as otherwise known bythose skilled in the art (including any and all equivalents), withoutdeparting from such principles.

1-16. (canceled)
 17. A Si powder having an average primary particle sizebetween 20 nm and 200 nm, wherein the powder has a SiO_(x) surface layerwith 0<x<2, the surface layer having an average thickness between 0.5 nmand 10 nm, and wherein the powder has a total oxygen content equal to orless than 3% by weight at room temperature.
 18. The Si powder of claim17, wherein the surface layer has a thickness between 0.5 nm and 5 nm.19. The Si powder of claim 17, having an oxidized SiO_(x) surface layerwith 1≤x<2.
 20. The Si powder of claim 17, having a metallic purity ofat least 98 wt % Si.
 21. The Si powder of claim 17, having a totaloxygen content less than 5% by weight after being aged for 1 hour at700° C. under atmospheric conditions and in air.
 22. The Si powder ofclaim 17, further comprising an element M selected from the groupconsisting of transition metals, metalloids, Group Ma elements andcarbon.
 23. A Li-ion battery comprising the Si powder of claim 17, as anegative electrode material.
 24. A powder comprising particles, whereinthe particles have an average primary particle size between 20 nm and200 nm, wherein the particles comprise a silicon core having a surfaceand a layer at the surface of the silicon core, wherein the layercomprises SiO, with 0<x<2 and has an average thickness between 0.5 nmand 10 nm, and the powder has a total oxygen content equal to or lessthan 3% by weight at room temperature.
 25. The powder of claim 24,wherein the layer at the surface of the silicon core consists of SiO_(x)with 0<x<2.
 26. The powder of claim 24, wherein the layer at the surfaceof the silicon core has a thickness between 0.5 nm and 5 nm.
 27. Thepowder of claim 24, wherein the layer at the surface of the silicon corecomprises SiO_(x) with 1≤x<2.
 28. The powder of claim 24, having ametallic purity of at least 98 wt % Si.
 29. The powder of claim 24,having a total oxygen content less than 5% by weight after being agedfor 1 hour at 700° C. under atmospheric conditions and in air.
 30. Thepowder of claim 24, further comprising an element M selected from thegroup consisting of transition metals, metalloids, Group Ma elements andcarbon.
 31. A Li-ion battery comprising the Si powder of claim 24, as anegative electrode material.